Global warming is already curbing gains in crop yields, melting ice and damaging coral reefs, a major UN study found on Monday.

The report by the U.N.’s Intergovernmental Panel on Climate Change (IPCC) was the second in a three-part review of climate change, documenting evidence, impacts and steps to cut greenhouse gas emissions.

Monday’s report focused on global impacts of rising temperatures, in a report released in Yokohama in Japan after a week-long review by world governments.

Impacts on the world economy still paled in comparison with economic growth, rising populations and advances in technology, the IPCC said.

Nevertheless, serious impacts from climate change were already visible, and these would get worse without cuts in greenhouse gas emissions.

Crops

At a global level, climate change was more frequently damaging than benefiting crops, the IPCC said, as a result of water stress, higher temperatures and extreme weather.

The report found negative impacts on global production of wheat and maize. Effects on rice and soybean yields were smaller.

Climate change had held back further crop yield advances, beyond those achieved until the 1990s, said Chris Field, one of the lead authors of the report.

Warming had benefited crop production in some high-latitude regions, such as in northern China and in Britain.

The report projected further damage, if temperatures continued to rise:

– “Without adaptation, local temperature increases in excess of about 1 degree Celsius above pre-industrial levels is projected to have negative effects on yields for the major crops, wheat, rice and maize, in both tropical and temperate regions” (global average surface temperatures have so far risen 0.85 degrees)

– “Under scenarios of high levels of warming, leading to local mean temperature increases of 3 to 4 degrees or higher, models based on current agricultural systems suggest large negative impacts… and substantial risks to global food production and security”

– “With or without adaptation, negative impacts on average yields become likely from the 2030s”

Elevated sea temperatures can cause mass coral bleaching and death, while local water pollution can reduce the chances for the coral to recover, the IPCC report said.

The IPCC documented widespread damage to corals in recent decades, linked both with the trend in rising sea temperatures, and particular warming events such as the regular El Nino weather phenomenon.

Average sea surface temperatures of the Indian, Atlantic and Pacific Oceans rose by 0.65, 0.41 and 0.31 degrees Celsius over the period 1950–2009, it found.

“The conclusion based on outputs from a wide range of emissions scenarios and models is that preserving more than 10 percent of coral reefs worldwide would require limiting warming to less than (an average of) 1.5 degrees compared to pre-industrial levels,” the report said.

Arctic

The Arctic has warmed at about twice the global rate since 1980, with positive and negative impacts.

Warming in Alaska had already forced indigenous peoples to relocate, the report said.

“Decreased sea ice thickness and extent, less predictable weather, severe storms, sea level rise, changing seasonal melt/freeze-up of rivers and lakes, changes in snow type and timing, increasing shrub growth, permafrost thaw, and storm-related erosion … are causing such severe loss of land in some regions that a number of Alaskan coastal villages are having to relocate entire communities.”

Thawing permafrost is a systemic threat to polar infrastructure including roads and buildings which can depend on permafrost for support.

In the future, large investments may be required to replace winter ice roads with land-based roads.

In some areas of Alaska, the winter road season has already decreased to 100 days from as much as 200 days in the 1970s, the IPCC reported.

The winter road network was projected to contract by an average 14% across eight polar nations by 2050, according to the IPCC.

The melting of permafrost may also lead to ground settlement which would undermine the stability of railways. Marine and freshwater transport may have to shift from ice routes to open-water or land-based alternatives.